* cygheap.h (cygheap_fdmanip::isopen): Set appropriate errno if fd not open.

* cygheap.h (cygheap_fdmanip::isopen): Set appropriate errno if fd not open.
* select.cc (fhandler_base::ready_for_read): Release an open guard mutex when
exiting with an error condition.
* syscalls.cc (_read): Check frequently for closed fd as a kludge until
something better is invented.